The mode of operation of electromagnetic actuators is based on the effect of the Lorentz force and the reluctance force (also called Maxwell's force).
Actuators, which are structured like a lifting magnet, can be utilized for the actuation of machine levers, valves, gate valves, switches, etc. Lifting magnets are electromagnets comprising armature, stator and coil/s. Their structure is simple and robust and they can generate great retaining powers with little power consumption. When lifting large loads, their electrical efficiency is small however, due to the large air gap associated with the heavy lift. In the most simple approximation (no stray field, no saturation), the current required for producing a specific force is proportional to the length of the air gap, and the power loss increases quadratically with the current. The actual ratios are even less favorable. Because of the high power loss, long-stroke lifting magnets can normally produce even only small initial forces (compared to the retaining force), if the electrical efficiency for the application is insignificant. The limit is given by the current rating. Lifting magnets are described as being “long-stroke,” for example, if the maximum lift h of the armature (relative to the stator) is in the order of magnitude h=sqrt(A), where A stands for the cross-sectional area of the armature. The quoted definition must only be understood as being a guide value, however. Generally speaking, to realize an approximate constant actuator force across the entire regulating distance is disproportionately more difficult for larger regulating distances than for smaller ones. The high retaining force is effective only if the air gap is almost zero.
By a suitable geometric design of armature and frame, the path-way performance curve of a lifting magnet can be influenced such (this is described as performance curve impact) that the reluctance force acting on the armature becomes almost independent of the path. Such types of actuators are described as “proportional magnets.” When the magnetic force of the armature acts against the restoring force of a spring, the position of the armature can be almost proportional to the armature current, if it is suitably configured. But proportional magnets supply only relatively small forces for long lifts. Moreover, in the attracted condition, proportional magnets can produce only comparatively small retaining forces (compared with lifting magnets without performance curve impact).
Another type of electromagnetic linear actuators are structured similar to a plunger coil, and are also described as electrodynamic actuators. When compared to lifting magnets, plunger coils are more delicate and more complex structural designs. Although suitably designed plunger coils are capable of producing almost uniformly large (Lorentz) forces, these must be absorbed from the free-standing and comparatively filigree coil, however. The cooling of plunger coils can also be technically challenging, since the coil must be suspended so that it can move and should be as light as possible in order to achieve high dynamics. (To mention an example, just think of an electrodynamic loudspeaker). For this reason, it can frequently not be firmly attached to a (solid) heatsink. Contrary to lifting magnets, plunger coils are moreover not capable of generating (retaining) forces using only low-power. They are not really suitable for applications in which it is necessary to maintain a large (retaining) force, using power consumption that is preferably as low as possible.
The object of the invention therefore consists in finding an electrical linear drive capable of producing retaining forces with a similar power like a lifting magnet (without impacting the performance curve), but which is also capable of producing a force in the order of magnitude of the retaining force with long lifts across the entire regulating distance.